While watching a couple of pro matches, I heard the commentators say that Timbersaw's pure damage is especially effective against strength heroes. Can anybody explain why?
What the commentators were actually referring to was the was one of the secondary effects of Timbersaw's first skill, Whirling Death. The effect is that if an enemy hero is hit, their primary attribute is reduced by 15%.
Since each point of strength is equal to 19 maximum hit points, Whirling Death effectively deals an extra .15 * 19 * the hero's strength points of damage to Strength heroes.
Personally the quote is a bit misleading.
"Timbersaw's pure damage is especially effective against strength heroes".
It's a mix of two mechanics:
Timbersaw does a lot of burst dmg (3 separate nukes within 0.5 sec), almost all pure - so it's a good hero against any kind of carry or tank - which are are almost all agi or str heroes. Because any magical defense items they commonly (might) make (plus the normal 25% base mdef) are rendered useless.
As mentioned in the earlier answer, timbersaw's first skill "Whirling Death" causes a temporary 15% reduction in stats - along with all his burst dmg, that hits str heroes hard who get most of their HP pool from str stats, not items - so on average it's a BONUS nuke of 12-13% of the str hero's total HP pool - which easily crosses > 200-300 dmg.